Discharge tube amplifier



March 7, 1939-. K. RATH v DISCHARGE TUBE AMPLIFIER Filed Jan. 21, 1951nuuuu'o'nuni INVENTOR Karl Raf-h LBY nrroRne r Patented Mar. 7, 19.39

PATENT OFFICE- mm m Ammrma assignments, to Radio as man. New York, N.

1., salmonl by meme Corporation of America,-

' aoomrationotneiaware I Application January 21, 1931, Serial No.510,296

11 Claims. (Cl. 178-171) My invention relates to apparatus for andmethods of operating electric amplifiers utilizing discharge devices,such as evacuated valves or radio tubes in a cascade arrangement and,

more particularly, to amplifiers having a direct connection of oneamplifying unit or valve with a succeeding amplifying unit or valve in acascade amplification system.

Amplifiers ofthis type are known as direct coupled or direct currentamplifiers and are primarily designed for the amplification of direct orunilateral electric current variations. This function makes it necessaryto use a direct coupling between the succeeding amplifying valves, sinceinductive or capacitative devices, if used as the coupling means betweenthe valves, would suppress the direct current component of a unilateralcurrent variation.

There is, however, another field where ampliflers with a direct couplingpresent meritorious advantages; namely, in the amplification of" purealternating currents, such as speech or microphone currents, picturesignal currents as used in picture telegraphy and television and thelike; in short, in all such cases where a faithful amplification andreproduction of the initial current-variations is essential and vitalfor the successful operation of an output or translating device operatedfrom the amplifier.-

However, direct coupled amplifiers tend to become unstable, whichrenders their use for amplifying current impractical.

Accordingly, an object of my invention is to provide a novel improveddirect coupled amplification system.

A further object of my invention is to provide novel direct coupledamplifying circuits, which. are stablein operation, efiicient and givefaithful reproduction for radio, telephone, talking picture signals inpublic address systems, picture telegraphy, television and the like.

. Still a further object of my invention is to provide a direct coupledamplification cascade system, which is substantially free from reactionof one amplifying unit or valve upon the preceding valve or valvesinsuring full stability of operation. 4

Another object of my invention is to provide means in connection with adirect coupled amplification system, using discharge valves in which thereaction of the current variations of one valve upon the biasingoperating voltage on a preceding valve or valves is such that a.substantially constant biasing. potential is main- 55 tained for suchpreceding valve or valves and a tendency of the biasing potential to"drift" is avoided.

These and further objects and features of my invention will become moreapparent as the following description proceeds, taken with reference 5to the accompanying drawing, in which I have shown, for purposes ofillustration, circuit diagrams illustrating a few embodiments of thenovel idea underlying the invention. I wish it to be understood,however, that the description 10 and exemplifications according to thedrawing should be regarded as illustrative only of the underlyingprinciple and the novel underlying idea of the invention, which, as willbecome obvious,

is subject to many modifications and variations 16 coming within itsbroader scope, as set forth in the appended claims.

Figure 1 represents a well-known direct coupled amplification circuit,which I have shown for the purpose of illustration only and clearer 20understanding of the novel features of the invention.

, Figure 2 shows a simple circuit arrangement of one form of embodimentof the invention.

Figure 3 is a similar system showing a complete operative circuit withan alternating current supply source.

Figure 4 illustrates an alternative system for practicing the invention,consisting of a structural combination of the novel parallel-feed type30 circuit in accordance with the invention and the older series-feed orstaggered type of circuit, as known in the art, and shownby Figure 1.

Figure 5 illustrates a further combined circuit arrangement comprisingthe system of the invention and a series-feed or staggered circuitarrangement, which forms the subject matter of my Patent No. 1,927,846,issued Sept. 26, 1933.

Referring more particularly to Figure 1, this illustrates a directcoupled amplification system 40 as hitherto known in the art, comprisingin the example shown three stages, each consisting of a vacuum valve l,2 and 3 respectively of usual construction and design, includingcathodes 4, 5 and 6, grid .electrodes 1, 8 and 9 and plate elec- 5trodes I II, II and I 2, respectively. A high voltage potential source,supplying a voltage equal to the sum of the anode voltages of theindividual valves, indicated by the plus and minus signs, is arranged inshunt to a potentiometer I5, from which are tapped suitable connectionsto the different electrodes of the amplifying valves. The gridelectrodes 8 and 9 of valves 2 and 3 are directly connected to theplates of the preceding valves l and 2 respectively and resistors l3 andll, called coupling resistors, are placed between the anode of thesevalves and suitable tap points.

shown at c and e on the potentiometer. The cathodes of the valves 2 and3 are also connected to potentiometer taps, as shown at b and d,displaced towards the negative side of the voltage drop on thepotentiometer in respect to the points and e respectively, in such amanner as to provide a suitable compensating potential between points band c and d and .e respectively, counteracting the plate voltage dropalong the resistors l3 and It thus removing the effect of the high platevoltages, which would otherwise be directly appliedto the'grids 8 and 9of tubes 2 and 3 and render operation impossible. The input has beenshown as applied directlyto the grid 1 of the first valve 6 and thenegative terminal point a on the potentiometer, whereby the inputcurrents are introduced at the terminals 18 andthe output or translatingdevice; such as loud speaker, relay, etc. inserted in the anode circuitof the third valve is connected to the output terminals at H.

The operation of this systemis as follows:- Assuming, for instance, avariation of' the grid cathode potential of the first tube, dependent onan incoming signal, this will produce, as the case may be, an increasedor decreased plate current draw through the resistor IS in the platecircuit and, accordingly, vary the potential applied to the grid 8 ofthe second tube, which, in its turn, will produce a similar variation ofthe plate current of the succeeding tube, which variation will beapplied to the translating device conmission of energy through thecircuit is blocked.

This is due to the fact that the plate current, such as, for instance,of tube 3 tracing the current from the plus tothe minus terminal of thehigh potential source, has to pass the part (1-1: of the potentiometerfrom which are tapped the operating potentials for the preceding valvesl and 2. If, for example, the plate current of valve 3 undergoes aehange, it will produce a corresponding change of the drop between d anda and, accordingly, shift or displace the potentials at the tap pointsa, b within this portion of the potentiometer, which will react on thegrids; 1 of valve 8, in such a manner as to allow the grid operatingpotential to drift toward unstable con-. ditions. As is obvious, thisdisadvantage becomes the more eifective the higher the number ofamplifying valves connected in cascade or, in other words, the greaterthe final plate current variations of the last valve or valves of thesystems, thus producing a cumulative reaction on the preceding valve orvalves, one of which may drift or shift its operating point to suchextremes on the operating characteristic-either towards its saturationvalue or towards its zero value-that it becomes inoperative and causesthe amplifier to entirely block the transmission of the currentvariations.

This disadvantageous condltioncould be prevented by making the currentthrough the potentiometer sumciently largeso that its order of"magnitude is comparatively high as compared to the order of magnitude ofthe plate current variations of the valves. "As these plate currents areusually of about 1 to milliamperes for ordinary amplifying valves 'andabout 20 to 30 milliamperes for power or output valves, it is necessaryto have at least one ampere current flow through the potentiometer toprevent appreciable reaction effect, provided furthermore there are onlya few amplifying stages-say about 2 or 3. In this way,

renders its application prohibitive for the purpose of broadcasting,radio, telephony, etc. Assuming, in the present case, an average platevoltage for each valve of about 100 volts, 2. high potential on thepotentiometer of about 300 volts would be required, which will result ina resistance for the potentiometer of 300 ohms, provided the requiredcurrent flow of 1 ampere, as above assumed. This will give a heat energyexpenditure in the potentiometer equal to 1 300=300 watts, which isexcessive, and would make .the operation too expensive and'uneconomical, especially for the purposes above stated, such asbroadcasting, television, etc. Therefore, lower potentiometer currentvalues have to be chosen, in which case, however, the reaction andinstability phenomena become appreciable and it is, therefore, theobject of the present invention to provide other means for rendering theamplifier stable and dependable in operation.

The new idea underlying the invention consists broadly in that aplurality of potentiometer circuits, each including an amplifying valve,are arranged substantially in parallel relationship to a common highpotential operating source and that the control or grid electrodes ofthe amplifying valves are connected to suitable potential points of thepreceding potentiometer circuit. This is basically distinct fromtheprior circuit arrangement above described, which uses a singlepotentiometer arrangement, from which are tapped all the amplifyingvalves, in a staggered fashion. I

A simple circuit diagram embodying this idea is shown by Figure 2. Thehigh potential which, in this case, is equal to the platepotentialrequired for one valve alone, is again shown to be suppliedfrom a source indicated by the plus and minus signs. The valves I, 2 and3 are all connected substantially in parallel to the potential supply,each in series with a suitable resistor or potentiometer l8, l9 and 20,respectively. Contrary to the prior arrangement, in which the plate isconnected to the grid of the succeeding valve, in the novel arrangementaccording to the invention the grid of one valve is connected to thecathode side of the preceding valve, as shown in Figure 2, according towhich the grids 8 and 9 of tubes 2 and 3 are connected to suitable tappoints I and g respectively on the series potentiometers l8 and I 9 forthe tubes i and 2. The input is again applied at terminals i6 betweenthe grid and cathode of the first valve, a suitable tap on thepotentiometer l8 being provided for adjusting the correct grid biasingpotential for valve I. The

I which the loud speaker present Figure 2, the output terminals beingdesignated in the drawing by the numeral II, to

or other translating device is to be connected.

The operation of this new system is as followsz-With no currentvariations being applied to the input terminal ii, a certain zero orrest ing operating plate. current will ilow through all of the valvesshunted across the plate current supply. The resistance of the seriespotentiometers ll, l8 and 20 is preferably of the order of theresistance of the valves, in which case the maximum potential dropchange will-take place on the potentiometer, dependent on a change ofwhich is exactly 3 volts below the potential of the cathode of valve I;that is, at a potential of 97 volts, so that, assuming equal potentialdrop through all the valves, thegrid '8 of the tube 2 will be at a 3volt negative potential in respect to its cathode 5, whichit is assumed,in the present example, corresponds to the proper and favorable gridbiasing operating potential on the rectilinear portion of the. gridvoltage-plate current operating characteristic curve of the valve. Thesame conditions hold for the valve 3, which receives its operating gridbias from potentiometer is of the preceding valve 2. Supposing now thatthe input currents supplied at terminals l6 increase the grid potentialof valve 1, then, as is obvious, this valve will drawan increasedcurrent and the drop across the potentiometer l8 will increase;

that is, the potential of the tap point will also be increased and,accordingly, the .grid potential of valve 2 also becomes more positive,whereby this valve will also draw an increased current, which is greatlyamplified as compared to the current through the valve I. The valve 2will act in the same manner on valve 3. As all the valves are connectedin shunt to the operating voltage potential source, it is clearly seenthat the currents from one valve do not aflect a preceding valve, as inthe case of Figure -1 and, accordingly,

it will be obvious the tendency of reaction and instability is overcome.Furthermore, a basic difference consists in that the potential values onthe potentiometers l8, I9 and 20 of the individual valves are beingdisplaced in the same direction; that is, either towards points ofincreasing or decreasing potential, so that the relative potentialdiiference applied between the grids remains substantially constant and'a drift .of the operating potential bias is prevented, which makes forcomplete stabilization and dependability of the circuit system. On theother hand, returning to Figure 1, from a simple theoretical analysisthat the potential points determining the grid-cathode bias of the tubesare displaced in oppositedirections, depending on changes of succeedingplate cu'rrents, which has the contrary effect to the system accordingto the invention shown by Figure 2; namely, of tending to cumulativelyincrease the tendency to become unstable and drift of the entire circuitsystem.

\ Referring to Figure 3, I have shown a complete wiring circuit of anamplifying system, which is essentially identical to Figure12 andcontains an e arrangement for direct operation from an alternatingcurrent source, as shown at 2|, such as, for instance, directly from analternating current house net work. The valves I, 2 and 3, in this case,are assumed to be equipped with filaments 4, B and l for alternatingcurrent heating by direct connection to the secondary windings 2|, 22and 28, respectively of an input transformer 24 supplied from the source25. The mid tap point of the secondaries 2i, 22 and 23, in

a well-known manner, serve as cathode connecting terminals. Theremaining parts of the circuit are identical to Figure 2. The highvoltage plate supply is provided through a rectifier 21 connected to afurther secondary winding 26 of the transformer 24 and a filter orsmoothing device of usual design, comprising parallel capacities 28 and28 and a series inductor 303 It is, of course, obvious that, in theplace of direct byalternating current or indirectly heated cathodes maybe used, of known construction, such as is shown in the followingFigures 4 and 5. It is, furthermore, possible to the cathodes, so-calledheating provide direct current heated cathodes, such as by connectingthe open ends of the filaments, according to Figure 2, to a suitablepoint on the potentiometers l8, l9 and 20, respectively.

The circuit in accordance with the invention presents a furtherimportant advantage, especially when an alternating current supplyarrangement is used, as illustrated in Figures 3, 4

and 5. As is well known, the direct currents as supplied from such anarrangement, in most cases, contain an alternating current ripple,,which produces a disturbing hum in the output device connected to theamplifier, such as loud speaker. In order to suppress this hum to aninappreciable value, it'would benecessary to use very bulky andexpensive smoothing arrangements, such as smoothing filters, asillustrated in Figures 3, 4 and 5. It was, therefore, found necessary toprovide further means, in addition to the usual filter devices, forminimizing or suppressing the eil'ect of the alternating current hum.For this purpose, it has been proposed to provide a neutralizing orcompensation arrangement, which consists in that a neutralizing orcounteracting potential is applied 'to the grid of one or a number ofthe amplifying valves, whifih counteracts the variations producing thehum note in the output of the amplifier. It is a special advantage ofthe circuit of my invention thatit does not need such additional meansand that it inherently acts to suppress any transmission oramplification of ripple or hum notes occurring in the direct currentpotential supplied from the rectifying system. .This is due to the factthat, with a change of the anode current, the drop in the individualpotentiometer shunt circuits comprised by the valves and their seriespotentiometers undergoes a change in the same direction, so that therelative potential diiference between grid and cathodes of the valvesremain substantially constant. Thishas already been pointed out aboveand is also the reason for the stability of the system.

Referring to Figure 2, it will be seen that, with a variation of theplate potential, the potential drop through the. circuit comprised by.valve l and potentiometer I 8 and the circuitcomprised by valve2 andpotentiometer i9 will vary by substantially the same amount and thepotentials between grid and cathode of valve 2 will be shifted in thesame direction, thus maintaining a constant relative value andpreventing the variations of the high-potential, due to a ripple or humcomponent, from aflectingyalve 2 and being amplified and transmitted tovalve 3 and to the output or translating device.

The new amplification system in accordance with the invention, such asdescribed hereinbefore, is subject to many variations and modifications, as is obvious, embodying the underlying inventive principle. One.of these modifications consists in a combination with a series feedsystem above referred to and as known in the art, according to Figure 1.

Figure 4 constitutes a combination of the circuits of Figuresl and 2,providing a common potentiometer I5 for the first valve and the twosucceeding valves, the latter being arranged accordingtothe parallelfeed arrangement of the invention. Accordingly, only one plate potentialtap point; namely,for valve 1 is provided at c, the valves 2 and 3 beingoperated in parallel relationship by'the same plate potential. The grid8 of the second valve is directly connected to the anode of the firstvalve, which includes a coupling resistor H in its plate circuitconnected to the tap point e on the potentiometer. The cathodes 5 and 6of the parallel valves 2 and 3 are joined together, each includingsuitable cathode lead series potentiometer resistors l9 and 20 and thecommon cathode terminal connected to the tap point b on thepotentiometer at a position towards the negative terminal relative tothe plate potential tap point 0. The

input is again applied to the grid and cathode of the first valve I,which is given a. suitable bias by shifting the tap point a of thecathode 4 in a positive sense on the potentiometer and the output isagain supplied from the terminal l1 connected in the common anode leadfor the parallel connected valves 2 and 3. I have furthermore shown inthis figure indirectly heated cathodes 4, 5 and 6, provided in thewell-known manner with separate heaters 32, 33 and 34, respectively,operated in parallel from the secondary winding 3| of an inputtransformer 24 supplied from an alternating current supply source 25.The high plate potential is provided in a manner similar to Figure 3through the further secondary winding 26 of the input transformer 24,rectifier 21 and smoothing arrangement.

If, in a system according to Figures 4 and 5, similar valves are used,the two last valves operating in parallel are equivalent to a powervalve of twice the capacity, producing high current variations necessaryfor operating the translating device, such as a loud speaker. It is, ofcourse, also possible to connect the first two valves in parallelarrangement and provide the third valve in series thereto, or any othermixed combination may be provided, as the particular case may require.

Referring to Figure 5, this represents a combination of a parallel feeddirect coupled amplification system, as shown by Figure 2, with a seriesfeed system, as forms the subject matter .of Patent 1,927,846, issuedSept. 26, 1933, consisting substantially of a Wheatstone-bridge couplingarrangement between one valve and a sucgrid bias potential variations inaccordance with the unbalancing of the bridge circuit produced by inputcurrent variations applied to the input valve. In its preferredembodiment, this system comprises at least two potentiometers l5 and 35,according'to Figure 5, connected across the high potential supplyterminals, whereby the plate electrodes and the cathode electrodes oisucceeding valves are connected to proper points,

each on one of said potentiometers in a stag-' gered fashion, in such amanner as to provide the proper grid bias operating potentials for thevalves, which, as in the usual circuit, have their plates directlycoupled to the grid electrodes of a succeeding valve. This is shown inFigure 5,

according to which the anode It ofvalve l is.

tial level difference :c, 11 between the points i and of thepotentiometers and i5 respectively corresponds to the proper operatinggrid biasing voltage required for the valve 8. The common plate terminalof the tubes 2 and3 is'connected to a proper tap it on the potentiometer35. The input is again applied atHi, whereby a proper grid bias for theinput valve l is procured by suitable taps l and h on the potentiometers35 and I5, respectively. The remaining elements and connections areidentical to Figure 4. Although, as has been pointed out, a series feedarrangement has a tendency to become unstable, it is readily understoodthat a'combined system, as per Figures 4 and 5, with only two effectiveseries stages, will give sufilcient stability and dependable results incases where only a few am-- plifylng stages are required.

1. In a cascade amplification system, comprising a first valve and asecond valve to becontrolled by said first valve, each of said valveshaving a cathode, a control electrode and a plate and being connectedto, a common potential source substantially in parallel relationship,individual potentiometer means in series with each of the cathode leadsof said valves and a direct current connection from the controlelectrode of said second valve to a. tap point on the potentiometer inthe cathode lead of said first valve.

2. In a cascade amplification system, comprising a first valve, a secondvalve to be controlled by said first valve, said valves having cathode,grid and plate electrodes, a common plate potential supply source,individual series potentiometer means in each of.the cathode leads ofsaid valves and a direct conductive coupling connection from the gridelectrode of said second valve to a tap point on the potentiometer ofsaid first valve.

3. In a cascade amplifier comprising a plurality of vacuum tubes havingcathode, anode and control electrodes, 9. common anode current supplysource, a. plurality of circuits connected to said source and arrangedin parallel relationship with respect to the source, each circuitincluding a potentiometer and one of said tubes, said tubes beingarranged at the positive ends of said circuits, conductive couplingconnections from the control electrodes of said tubes to a point on thepotentiometer circuit including the preceding tube and means forapplying input voltage vari- 8,149,861 a I I 5 8.In-acascadeampiifierasdescribedinclaim.

ations to the control electrode of'the first of said 4. In a cascadeamplifier, the combination of a plurality of amplifying valves eachhaving cathode, anode and grid electrodes, a common anode current supplysource, a plurality of circuits connected to said source and arranged inparallel relationship with respect to the source, each including apotentiometer and one of said valves, said valves being arranged at thepositive ends of said circuits, direct coupling connections from thecontrol electrodes of said valves to a point on the cathode side of thepotentiometer circuit including the preceding valve, means for applyinginput voltage variations to the grid of the'first valve and furthermeans for connecting a translating device in the common anode supplylead for said valves.

5. In a cascade amplifier comprising a plurality of amplifying unitseach having cathode, grid,

and plate electrode; 'a common plate potential supply source for saidunits, the anodes of said units being connected to the positive pole ofsaid source and the cathodes of said units being connected to thenegative pole of said source; in-

dividual biasing resistance means in each of the cathode supply leads ofsaid units; and a direct conductive coupling connection from a tap pointon said biasing means to the grid of the succeeding amplifying unit. a

6. In a cascade amplifier comprising a plurality of thermionicamplifying units each having cathode, grid and plate electrodes; acommon plate potential supply source; individual biasing potentiometersconnected to the cathode of said am y ng units to form individual branchcircuits therewith connected across said source-in substantial parallelrelationship; and a direct conductive coupling connection from the gridof said amplifying units to a .point on the potentiometer in thepreceding branch circuit including the preceding amplifying circuit.

7. In a cascade amplifier as described in claim 6. means to render thepotentials of the cathode ofaaldimitsindependentoi'thecathcdeheatlng 6in which the cathodes of the, alternating current heated type are usedwith a separate heating transformer secondary winding for eachindividual cathode.

9. In a cascade amplifier as described in claim 6 in which units withindirectly heated cathodes are provided for said amplifying units.

10. An amplifier comprising a first amplifying unit; a second amplifyingunit, each having a cathode, anode and control electrode; a commonanodepotential source; biasing impedance means connected to the cathodes ofsaid units to form independent branch circuits together wlthsaid unitsconnected across said source in substantial parallel relationship; and adirect coupling connection from the control electrode of said secondunit to an intermediate point of said impedance means, said impedancemeans being designed to secure proper biasing potential for the controlelectrode of said second unit relative to its cathode. a v

11. An amplifying device comprising in combination, first, second andthird amplifying tubes each having a filament, a control grid and ananode, a transformer having separate secondary windings connected tosaid filaments, a source of positive potential, a connection betweensaid anodes and the positive terminal of said source, a resistorconnected to the midpoint of the secondary winding of the first'tubefilament and to the negative terminal of said potential source,connections between the grids of the first and second tubes and spacedapart points on said resistor, a second resistor connected to themidpointof the secondary winding of the second tube filament and thenegative terminal of said po-' tial source.

' KARL RL'I'H.

